Method for producing homogeneous material with an extruder, an extruder, and a multilayer plastic pipe

ABSTRACT

A method for producing a homogeneous material with an extruder is provided. The extruder has at least two annular conical feed gaps situated between a rotatable rotor and a stator so that the material to be extruded is supplied between the rotor and the stator. Between the delivery point of the material to be extruded and the nozzle of the extruder in the direction of travel of the material the at least one rotor or stator of the extruder is provided with openings passing therethrough so that at least some of the material to be extruded is made to flow through the openings from one annular conical feed gap to another annular conical feed gap. The invention also relates to an extruder and to a multilayer plastic pipe.

The invention relates to a method for producing homogenous material withan extruder at least two annular conical feed gaps situated between arotatable rotor and a stator so that the material to be extruded issupplied between the rotor and the stator.

The invention further relates to an extruder comprising at least twoannular conical feed gaps situated between a rotatable rotor and astator, and which comprises means for supplying the material to beextruded between the rotor and the stator.

The invention also relates to a multilayer plastic pipe comprising aninner and outer pipe and an intermediate layer formed of softer materialand situated between the inner and outer pipe.

U.S. publication 3,314,108 discloses an extruder comprising two conicalstators and a conical rotor that is placed rotatably between thestators. The plastic material to be extruded is supplied via twoseparate conduits to different sides of the rotor. The rotor is providedwith screw-shaped grooves by means of which the material is transferredtowards an orifice at the narrower end of the extruder. The intermixingof the materials to be fed via different conduits is prevented withseals at the end where the material is supplied, wherefore the materialssituated on different sides of the rotor only come together after therotor near the end of the extruder, wherefore the intermixing of thematerials remains insignificant. Further, unequal pressures may easilyoccur on different sides of the rotor due to the feed rates of theseparate material flows fed from different conduits, and the pressuredifference stresses the rotor and may damage it.

European patent application 89 906 779.7 discloses an extrudercomprising several conical stators, and conical rotors that arerotatably placed between the stators. The material to be extruded issupplied along one conduit to the beginning of each rotor, whereupon theinlet of the rotor comprises openings via which the material can alsoflow to the other side of the rotor. By means of grooves provided oneach side of the rotor, the material is transferred towards the end ofthe extruder. Also in this apparatus, the material flows passing ondifferent sides of the rotor come together only near the orifice,wherefore the materials situated on different sides do not intermixefficiently. Further, there may be a great difference in pressure ondifferent sides of the rotor and it stresses the rotor and may damageit.

When multilayer pipes are prepared, the intermediate layer or layers aretypically made less strong than the outer and inner layers, or lessstrong materials are commonly used in the intermediate layer compared tothe outer and inner layers. The material of the intermediate layer ispreferably for example waste plastic, since this arrangement decreasesthe pollution of the environment and is also very advantageouseconomically. Another alternative is the foaming of the plastic materialof the intermediate layer, but the foaming can naturally also beperformed on waste plastic. However, especially the use of waste plasticrequires that the equipment to be used has better mixing and processingproperties than usually. Also, it may be necessary to mix reinforcingfibres in the intermediate layer, and the even distribution of thefibres in this layer requires very efficient mixing.

The extrusion apparatuses for multilayer pipes that have been developedso far have not provided sufficient mixing of the intermediate layers inthe aforementioned cases.

U.S. Pat. No. 4,364,882 discloses a conventional foamed pipe made ofPVC. The PVC is foamed to the density of 500 kg/m³, which in factrepresents the minimum that can be achieved with the conventionaltechnology. A typical foaming degree for PVC foam is 57%, in which casethe density is 800 kg/m³, since with lower values the strengthproperties of the PVC foam deteriorate rapidly. The patent discloses apipe having an outer diameter of 315 mm and the following structure:

the inner pipe has a thickness of 1.25 mm, the intermediate layer has athickness of 9 mm and the outer pipe 1.25 mm. The total thickness of thepipe is thus 11.5 mm, the total weight is 7.63 kg/m and the obtainedsaving in weight compared to a conventional pipe having correspondingrigidity is 29%.

As the above example shows, foamed plastic provides considerable savingin material (i.e. in costs) while producing a lighter pipe structure.The use of such "inferior" material as an intermediate layer isappropriate in this connection, since this layer is the least subjectedto mechanical stresses, such as wearing and stress deformations, and tophysical and chemical stresses, such as UV radiation and otherimpurities.

On the other hand, when the foaming degree of the intermediate layer isincreased, i.e. the density is decreased, the properties of the foamedmaterial also deteriorate significantly. Until now the maximum foamingdegree used has been in practice such that it has decreased the densityof the material into half of the density of unfoamed material, sinceafter this level the strength of the foam deteriorates significantly,and the construction of a pipe with a strong structure has thus beenconsidered impossible.

The above-described known multilayer pipes with a foamed intermediatelayer have several disadvantageous properties, the most important ofwhich is low impact resistance. The reasons for such poor mechanicalstrength include for example the poor homogenization of the plasticmaterial.

The purpose of the present invention is to provide a method forproducing homogenous material with an extruder, and an extruder in whichthe material supplied thereto can be mixed efficiently and whichcomprises none of the aforementioned problems.

Another purpose of the invention is to improve known multilayer pipes.

The method according to the invention is characterized in that at leastone rotor or stator comprises openings between the delivery point of thematerial to be extruded and the nozzle of the extruder in the directionof travel of the material, so that at least some of the material to beextruded can be made to flow through the openings.

Further, the extruder according to the invention is characterized inthat at least one rotor or stator comprises openings between thedelivery point of the material to be extruded and the nozzle of theextruder in the direction of travel of the material, at least some ofthe material to be extruded being able to flow through the openings.

Further, the multilayer pipe according to the invention is characterizedin that at least one layer consists of fibre-containing solid or foamedreprocessed plastic.

The manufacture of the pipe utilizes the method and apparatus accordingto the invention.

The essential idea of the invention is that between the conical annularfeed gaps there is a rotor or a stator comprising openings via which thematerial situated on different feed gaps can flow. Further, the idea ofa preferred embodiment is that the rotor and the stators comprise ovalnotches that are positioned in such a way that they partly overlap sothat in these notches the material supplied to the extruder can bemixed. The idea of another preferred embodiment is that severaldifferent agents are supplied to the extruder and they can beintermixed. The idea of a third preferred embodiment is that thematerial to be extruded is fed into the narrow end of the rotor, and thegrooves of the rotor transport the material along the first side of therotor to the wide end of the rotor where the material can enter theother side of the rotor via the openings provided in the rotor,whereafter the rotor grooves transport the material along the other sideof the rotor and out via the nozzle of the exturder.

The invention has the advantage that the material to be supplied can bemixed efficiently, whereupon very homogenous material can be dischargedfrom the extruder. A further advantage is that the openings provided inthe rotor or stator between the feed gaps balance the pressures ondifferent feed gaps, whereupon material can be supplied with differentpressures to different feed gaps, for example. The advantage of anotherembodiment is that the oval notches provide even more efficient mixingof the material. By means of the invention, several different agents canbe mixed and an efficiently blended homogenous mixture can be dischargedfrom the extruder. By supplying the material to the narrow end of therotor, it is possible to provide a long mixing section, thus producing avery homogenous mixture. Further, it is possible to utilize the overheatgenerated during the final stage of the extrusion to melt the materialsupplied to the extruder. Another advantage is that the removal of gasand moisture from the material to be extruded can be arranged in theextruder at the point where the material is transferred via the openingsin the rotor to the other side of the rotor.

Especially good mechanical properties of a multilayer plastic pipe areprovided by the embodiment of the invention where the outer pipe and theinner pipe are reinforced by means of orientation and/or orientedfibres. Pipes that have been extruded in a conventional manner do notusually employ staple fibres for reinforcing the pipe mainly becauseduring the extrusion the fibres are oriented according to the pipe axisand the reinforcing characteristic of the fibres is of no use. Thisconcerns both pressure pipes and sewage pipes that require reinforcementin the circumferential direction. With certain techniques, such asrotating mandrels or rotating nozzles, the fibres can be oriented partlyin the circumferential direction in the surface layers of the pipe bymeans of discontinuing the rotating motion. The use of this kind ofdefiberization in thermoplastic pipes for sewage is completely unknowndue to the high costs of the process, for example. Fibre-containingmasterbatch is typically about 3 to 4 times more expensive thanconventional polyethylene. Adding fibres thus increases the price of thepipe, since the increased strength properties are not sufficient tocompensate for the higher price of the raw material. According to theinvention, it has surprisingly been found that the adding of fibres doesproduce a less expensive final product than the conventional pipes. Thisis due to the fact that the reinforcing effect of the fibres is great inthe layer that bears most of the load in the pipe according to theinvention, i.e. in the inner and outer pipe, and for example inunderground sewage and drainage pipes which are designed by using thering rigidity as the criterion, the intermediate layer of the pipe isnot used efficiently.

The invention will be described in greater detail in the accompanyingdrawings, in which

FIG. 1 is a schematic side view, in cross-section, of an extruderaccording to the invention,

FIG. 2 shows a part of a rotor in the extruder of FIG. 1,

FIG. 3 shows a detail of the extruder of FIG. 1,

FIG. 4 is a schematic side view, in cross-section, of a second extruderaccording to the invention,

FIG. 5 is a schematic side view, in cross-section, of a third extruderaccording to the invention,

FIG. 6 is a schematic side view, in cross-section, of a fourth extruderaccording to the invention,

FIG. 7 is a side view of a rotor in the extruder of FIG. 4,

FIG. 8 is a schematic side view, in cross-section, of a fifth embodimentof an extruder according to the invention,

FIG. 9 is a schematic side view, in cross-section, of a sixth embodimentof an extruder according to the invention,

FIG. 10 is a schematic side view, in cross-section, of a seventhembodiment of an extruder according to the invention,

FIG. 11 is a schematic side view, in cross-section, of an eighthembodiment of an extruder according to the invention, and

FIG. 12 shows a detail of a supply conduit in the extruder of FIG. 5.

FIG. 1 is a cross-sectional side view of an extruder according to theinvention. The extruder comprises an inner stator 1 and an outer stator2 placed outside the inner stator. At least the outer surface of theinner stator 1 and the inner surface of the outer stator 2 are conical.Between the inner stator 1 and the outer stator 2 there is a conicalrotor 3. Then there is an annular conical feed gap on both sides of therotor 3. The rotor 3 is adapted to move rotatably between the innerstator 1 and the outer stator 2. The rotor 3 is rotated by a motor 5.The motor S may be for example a hydraulic motor, an electric motor orsome other motor that is known per se and that is suitable for thepurpose. The motor 5 is adapted to rotate the rotor 3 through a gearsystem 4. The speed of rotation of the rotor 3 can be adjusted in adesired manner by means of the gear system 4. On the other hand, forexample when an electric motor is used, the gear system 4 is notnecessary, since the rotational frequency of the rotor 3 can be adjustedeasily by regulating the rotational speed of the motor 5 in a mannerknown per se. The above-described components of the extruder are knownper se, wherefore they have not been discussed in greater detail in thisconnection.

The extruder further comprises a first supply conduit 6 along which thematerial to be extruded can be fed into the exterior of the rotor 3between the rotor 3 and the outer stator 2. The extruder also comprisesa second supply conduit 7 along which material can be fed into theinterior of the rotor 3 between the rotor 3 and the inner stator 1 viaan opening or openings 8 provided in the rotor 3. The material to be fedinto the first supply conduit 6 is supplied with a first feeding device9. Correspondingly, for the purpose of feeding materials into the secondsupply conduit 7, the arrangement comprises a second feeding device 10.The feeding devices 9 and 10 can be for example feed screws, pumps orsome other devices known per se. With this feeding device, the flow rateof the material to be fed into the supply conduit can be adjusted.

The rotor 3 comprises on the outside outer grooves 11a and on the insideinner grooves 11b. The grooves 11a and 11b are screw-shaped, whereuponduring the rotation of the rotor 3 the grooves 11a and 11b transport thematerial to be extruded towards the nozzle section of the extruder. Therotor 3 further comprises openings 12 via which the material to beextruded can flow from the exterior of the rotor 3 into its interior andvice versa. therefore the materials situated on the outside and insideof the rotor 3 can be intermixed already inside the rotor 3, whereuponthe result is a well mixed material. The stators 1 and 2 furthercomprise notches 13, whereupon as the material to be extruded arrives atthe notches 13, its flow does not continue towards the nozzle of theextruder as evenly as before, but at these notches the material is mixedfurther. At the notches 13 there are also corresponding notches in therotor 3. The notches 13 are made so large and placed at such intervalsfrom one another that the notches of the rotor 3 and the stators 1 and 2overlap at least partly.

FIG. 2 shows a part of the rotor of the extruder of FIG. 1. Thereference numerals in FIG. 2 correspond to those in FIG. 1. The notches13 of the rotor 3 are oval in shape and they are placed diagonally insuch a way that as the rotor 3 rotates, the notches 13 transport thematerial to be extruded towards the nozzle of the extruder. Some of thecorresponding notches 13 of the stator are denoted with a broken line.The notches 13 of the rotor 3 and the stator are positioned diagonallyin opposite directions and they overlap partly, so that the material ismixed efficiently in the notches 13.

FIG. 3 shows a detail which includes a part of the rotor 3 and a part ofthe outer stator 2. The ference numerals in FIG. 3 correspond to thosein FIGS. 1 and 2. The rotor 3 and the outer stator 2 are spaced in sucha way that there remains a gap 15 between the necks 14 situated betweenthe outer grooves 11a of the rotor 3, and the outer stator 2. In such acase, the outer grooves 11a transport the material to be extrudedupwards as seen in the figure, but some of the material can flowdownwards in the figure through the gap 15, whereupon the mixing of thematerials is improved further. There may naturally be a similar gapbetween the rotor 3 and the inner stator 1.

FIG. 4 shows a second extruder according to the invention. The referencenumerals in FIG. 4 correspond to those in FIGS. 1 to 3. The supplyconduit 6 and the feeding device 9 connected thereto are placed at thenarrower end of the rotor 3. The grooves of the rotor 3 are placed insuch a way that as the rotor 3 rotates, the grooves transport thematerial to be supplied between the rotor 3 and the outer stator 2upwards in the figure. When the material arrives at the openings 12, itcan flow to the interior of the rotor 3 via the openings 12, and theinner grooves of the rotor 3 are arranged to press the material to beextruded downwards as seen in the figure. The movement of the materialto be extruded is illustrated by arrows in FIG. 4. For the sake ofclarity, FIG. 4 does not show the rotor grooves nor the possible notchesof the stators and the rotor. The grooves transporting the material maybe situated to the stator as well. Preferably the grooves and theopenings 12 should be constructed in such a manner that at each momentas the rotor rotates the sum of the areas of the rotor openings 12visible at the end of the groove remains substantially constant. Thenthe material transported by the grooves can flow evenly from the groovesvia the rotor opening 12, whereupon there will be no pumping effect inthe flow of material. When the material to be extruded is pressedoutwards between the rotor 3 and the outer stator 2, it is subjected topressure. When the material thereafter arrives at the openings 12, thepressure acting on the material is removed, and gases and moisturepossibly found in the material can be removed via a discharge outlet 16.The discharge outlet 16 can be made to pass through the outer stator 2,as shown in FIG. 4, or through the inner stator 1, whereupon the meansfor removing moisture and gas can be preferably placed inside theextruder. There may be more than one supply conduit 6 and feeding device9, whereupon different feeding devices 9 can be used to supply forexample different materials from different places or the flow of thematerial to be supplied to the extruder can be balanced in some othermanner, if desired. The arrangement of FIG. 4 makes it possible toincrease the distance the material to be extruded travels in theextruder, whereupon the mixing of the material can be implementedeffectively. As the material to be supplied travels upwards between therotor 3 and the outer stator 2, the friction generates heat which istransmitted through the rotor 3 to warm the material between the rotor 3and the inner stator 1. Therefore this heat that would normally becooled and thus completely wasted can be utilized efficiently.

FIG. 5 is a side view, in cross-section, of a third extruder accordingto the invention. The reference numerals in FIG. 5 correspond to thosein FIGS. 1 to 4. In the extruder of FIG. 5, a supply conduit/conduits 6and correspondingly a feeding device/devices 9 are positioned frominside the extruder through the inner stator 1 to supply the material tobe extruded between the inner stator 1 and the rotor 3. The rotor 3grooves must naturally be placed in such a way that as the rotor 3rotates, the material to be extruded is first transported upwards, asseen in the figure, between the inner stator 1 and the rotor 3, andafter it has passed through the opening 12 to the outside of the rotor3, the material is transferred downwards in the figure between the rotor3 and the outer stator 2, as shown with arrows in FIG. 5. By positioningthe supply conduit(s) 6 from the inside of the extruder, the structureof the extruder can be made compact. Further, the overheat generatedduring the extrusion can thus be utilized for preheating, in the supplyconduit 6, the material to be supplied.

FIG. 6 is a side view, in cross-section, of a fourth extruder accordingto the invention. The reference numerals in FIG. 6 correspond to thosein FIGS. 1 to 5. The extruder according to FIG. 6 comprises two rotors,an inner rotor 3a and an outer rotor 3b. The gear system 4 is positionedin such a way that by means of the system the inner rotor 3a is rotatedin a different direction than the outer rotor 3b. The grooves of therotors 3a and 3b are placed in such a way that the grooves of the outerrotor 3b transport the material to be supplied by the first feedingdevice 9 via the first supply conduit 6 between the outer rotor 3b andthe outer stator 2 upwards as seen in the figure. Correspondingly, thegrooves of the inner rotor 3a transport the material to be supplied bythe second feeding device 10 via the second supply conduit 7 between theinner rotor 3a and the inner stator 1 upwards in the figure. When thematerial arrives at the openings 12, it passes through the openings 12into the space between the inner rotor 3a and the outer rotor 3b. Theouter grooves of the inner rotor 3a and correspondingly the innergrooves of the outer rotor 3b are positioned in such a way that thematerial between the rotors 3a and 3b moves downwards as seen in thefigure by the action of the grooves and further out via the nozzle ofthe extruder. The movement of the materials is described with arrows inthe accompanying figure. The materials to be supplied to different sidesof the rotors 3a and 3b thus come together between the rotors 3a and 3b,whereupon they are efficiently intermixed. There may naturally be anintermediate stator between the inner rotor 3a and the outer rotor 3b,so that the materials flow separately between the inner rotor 3a and theintermediate stator and between the outer rotor 3b and the intermediatestator, and they only come together after the intermediate stator.

FIG. 7 is a side view of the rotor 3 in the extruder of FIG. 4. Thereference numerals in FIG. 7 correspond to those in FIGS. 1 to 6. Thedirection of rotation of the rotor 3 is shown by the arrow A. The outergrooves 11a of the rotor 3 thus transport the material to be extrudedupwards as seen in the figure. The upper parts of the grooves 11a aremade to end at the openings 12, whereupon the material is transferredvia the openings 12 to the interior of the rotor 3. The number of theopenings 12 must be at least equal to that of the outer grooves 11a.Since the rotor 3 is positioned with its wider end upwards, the materialto be extruded remains at the lower part of the opening 12, whereforedischarge outlets 16 can be placed at the upper parts of the openings 12for discharging gas and moisture.

FIG. 8 is a side view, in cross-section, of a fifth extruder accordingto the invention. The reference numerals in FIG. 8 correspond to thosein FIGS. 1 to 7. The extruder of FIG. 8 comprises a conical feed rotor17 that is rotated by a rotating shaft 18. The rotating shaft 18 is alsoconnected to a feed screw 19 by means of which the material suppliedinside the feed rotor can be fed into the interior of the extruder. Theextruder also comprises a supply conduit 6' into which a second materialto be supplied is fed by a feeding device 9'. The surface of the feedrotor 17 situated on the side of the inner stator comprises grooves,whereupon as the feed rotor 17 rotates, the grooves transport the secondmaterial to be supplied from the supply conduit 6' to the interior ofthe extruder. The grooves of the feed rotor 17 are not shown in FIG. 8for the sake of clarity. The first material that is supplied to the feedrotor 17 can be for example a filling agent, and the material suppliedfrom the supply conduit 6' can be for example plastic. The feed screw 19can be made easily changeable, so that the amount of filling agentsupplied can be adjusted by the screw according to the desiredproportion of filling agent. As the plastic supplied from the supplyconduit 6' travels between the feed rotor 17 and the inner stator 1, thefriction generates heat that warms the material inside the feed rotor17. This feed rotor 17 arrangement prevents the hanging up of thematerial situated inside the rotor. The materials to be supplied cometogether near the central shaft 19 from where they are conducted bymeans of the grooves provided in the rotor 3 upwards in FIG. 7 in themanner shown by the arrows in the figure between the rotor 3 and theinner stator 1. The rotor 3 grooves are not shown in FIG. 7 for the sakeof clarity. The rotor 3 and the inner stator 1 may comprise notches 13for improving the mixing of the material. At least a part of thematerial can pass through the openings 12 provided in the rotor 3 to thespace between the rotor 3 and the outer stator 2. Either all thematerial can be made to pass through the openings 12 or a part of thematerial may circulate around the upper end of the rotor 3 as shown inFIG. 7 to the space between the rotor 3 and the outer stator 2. The heatgenerated by the friction resulting from the grinding of the materialbetween the feed rotor 17 and the inner stator 1 is also transmittedthrough the inner stator 1 to warm the mixing section situated betweenthe rotor 3 and the inner stator 1. The extruder further comprises asecond supply conduit 7 and a second feeding device 10 along whichconduit 7 material can be fed between the rotor 3 and the outer stator 2near the nozzle of the extruder. The material can be for exampleadditive at can't stand a too long residence time caused for example bynotches 13. The material can be for example masterbatch type plasticthat contains for example peroxide whereby the material is verysensitive to higher temperature.

FIG. 9 is a side view, in cross-section, of a sixth extruder accordingto the invention. The reference numerals in FIG. 9 correspond to thosein FIGS. 1 to 8. The extruder of FIG. 9 comprises a rotating shaft 18that is arranged to rotate the feed screw 19 by means of which materialcan be supplied to the interior of the extruder. From the lower end ofthe feed screw 19 the material can flow between the rotor 3 and theinner stator 1 via an opening 20 provided in the inner stator 1. Inconnection with the feed screw 19 there is a force-feed spiral 21 havinga thread that is opposite in direction to the threads of the feed screw19. In such a case, the force-feed spiral creates a pressure in thematerial to be supplied, whereupon the material can be forced downwardsin the figure with the freed screw 19. In the middle of the rotatingshaft 18 there is a duct 22 so that the extruder can also be used forcoating cables, for example.

The rotor 3 is mounted in bearings into the gearing frame 25. Thegearing frame 25 is connected to the stators 1 and 2 with a fasteningbolt 23. Between the gearing frame 25 and the stators 1 and 2 there arespring means, for example plate springs 24, so that the gearing frame 25and therefore also the rotor 3 can move to some extent in the axialdirection of the exturder with respect to the stators 1 and 2. If forexample the pressure between the rotor 3 and the inner stator 1increases, the plate springs 24 situated on the opposite side yield, sothat the gap between the inner stator 1 and the rotor 3 grows and thefriction increases. In such a case, the material warms up andsimultaneously becomes softer, whereupon the pressure decreases and thegap between the rotor 3 and the inner stator 1 is correspondinglyreduced. When the pressure between the rotor 3 and the outer stator 2increases, the result is naturally opposite. Therefore this arrangementprovides a self-adjusting gap. FIG. 9 also shows seals 26 that preventthe material to be supplied from entering the rotating mechanism of therotor 3.

The discharge outlet 16 is arranged to conduct gases and moisture outvia the material to be fed with the feed screw 19. If the opening 12 isfilled by the material to be extruded to such an extent that some of thematerial can enter the discharge outlet 16, this is not harmful sincethe material can be conducted along the discharge outlet 16 back to thematerial to be supplied.

The rotor 3 grooves that transport the material to be extruded upwardsinside the rotor 3 as seen in FIG. 9 and downwards outside the rotor 3as shown in FIG. 9 according to the arrows are not depicted in FIG. 9for the sake of clarity. The steepness of the grooves determines howsharply the corners of the grooves grind the material passing throughthe opening 20.

FIG. 10 is a side view, in cross-section, of a seventh extruderaccording to the invention. The reference numerals in FIG. 10 correspondto those in FIGS. 1 to 9. The extruder of FIG. 10 comprises threerotors: an inner rotor 3a, an outer rotor 3b and an intermediate rotor3c. The rotating means of the rotors are not shown in the figure for thesake of clarity. Between the inner rotor 3a and the intermediate rotor3c there is a first intermediate stator 27, and between the outer rotor3b and the intermediate rotor 3c there is a second intermediate stator28. Material is supplied along the first supply conduit 6 between theinner rotor 3a and the inner stator 1. The material forms the innermostlayer of the product to be extruded, i.e. a so-called inner skin.Material is supplied along the second supply conduit 7 between the innerrotor 3a and the first intermediate stator 27. This material can be forexample reprocessed plastic. Material is fed along the third supplyconduit 29 between the outer rotor 3b and the second intermediate stator28. The material can be for example adhesion plastic. Material issupplied along the fourth supply conduit 30 between the outer rotor 3band the outer stator 2. The material forms the outer layer of theproduct to be extruded, i.e. a so-called outer skin. The reprocessedplastic to be supplied along the second supply conduit 7 has oftencoarse and uneven granulation, whereupon the grooves 11 of the innerrotor 3a can be made relatively large, if necessary, so that thematerial that is difficult to process can be transported forward bymeans of the rotor 3a. The material is first transported between theinner rotor 3a and the first intermediate stator 27 towards the narrowerend of the inner rotor 3a where the material is passed via an aperture31 through the intermediate stator 27 between the intermediate rotor 3cand the first intermediate stator 27. The grooves situated inside theintermediate rotor 3c are arranged to conduct the material towards thewider end of the intermediate rotor 3c where the material can flow viathe opening 12 to the exterior of the intermediate rotor 3c and furtherout from the nozzle of the extruder. FIG. 10 does not show the groovesof the rotors 3a to 3c for the sake of clarity, except for the grooves11 situated outside the inner rotor 3a. The passage of the materials tobe supplied is clarified by means of arrows in FIG. 10. The discharge ofgas and moisture via the discharge outlet 16 is arranged to pass outthrough the first intermediate stator 27 and via the material to besupplied along the second supply conduit 7 between the inner rotor 3aand the first intermediate stator 27. Preferably can the properties ofthe well-mixed plastic be improved by chemical cross linking using forexample peroxides or azocompounds. In that case the product will not becross-linked until after the extrusion. The strength of this kind ofhomogenous material with filling agent is essentially better than thestrength of a cross-linked material not containing filling agentTypically cross-linked plastics have very high molar mass whereby themixing of filling agents is very difficult. However by using the methodand apparatus according to the invention the mixing could be made.Typically the degree of cross linking in pipes used as hot water pipesshould be very high. However when using reprocessed plastic the degreeof cross-linking don't have to be that high.

FIG. 11 is a side view, in cross-section, of an eighth extruderaccording to the invention. The reference numerals in FIG. 11 correspondto those of FIGS. 1 to 10. In the extruder of FIG. 11, material issupplied via the first supply conduit 6 and the third supply conduit 29between the outer stator 2 and the outer rotor 3b. Correspondingly, thesecond supply conduit 7 and the fourth supply conduit 30 supply thematerial to be extruded to the space between the inner stator 1 and theinner rotor 3a. The material is naturally supplied to the third supplyconduit with a third feeding device 32 and to the fourth supply conduitwith a fourth feeding device 33. The grooves provided on the outside ofthe outer rotor 3b are placed in such a way that the material to besupplied from the first supply conduit 6 is conducted upwards as 35 seenin FIG. 11 towards the wider end of the outer rotor 3b. The material tobe fed along the third supply conduit 29 is conducted with oppositegrooves towards the nozzle of the extruder. Correspondingly, thematerial to be fed along the second supply conduit 7 is first conductedtowards the wider end of the inner rotor 3a, and the material to besupplied along the fourth supply conduit 30 is conducted towards thenozzle of the extruder. For the sake of clarity, the grooves of therotors 3a and 3b are not shown in the figure, but the directions oftravel of the material are shown with arrows. With this arrangement, theouter skin of the product to be extruded, i.e. the outer pipe Eo, can besupplied along the supply conduit 29, and the material to be fed alongthe fourth supply conduit 30 can be formed into the inner skin of theproduct, i.e. the inner pipe Ei. The materials supplied along the firstsupply conduit 6 and the second supply conduit 7 can be mixed veryefficiently and they form the intermediate layer Ef. In the case of FIG.11, the outer pipe Eo, the inner pipe Ei and the intermediate layer Efare formed in one and the same thermic unit, which results in smallerlosses and lower costs than previously.

The outlet end of the extruder comprises a central annular extrusionconduit 35 for discharged plastic material, i.e. a plastic pipe preformE. The opposite surfaces of the rotors 3a and 3b comprise spiral groovesthat are positioned crosswise with respect to each other, i.e. they aredifferently handed. By means of the grooves, the plastic materialsupplied between the rotors can be foamed very effectively. If theplastic is waste plastic, it can also be simultaneously processed andmixed vigorously. When the plastic is waste plastic, it is also possibleto mix reinforcing fibres therein, if desired.

The extruder also comprises means for spraying gas in the space betweenthe rotors 3a and 3b. The means are not shown in the figure for the sakeof clarity. The gas may be for example nitrogen or carbon dioxide. Thegas is sprayed in a point where the plastic material has substantiallyalready melted and the gas is mixed with the plastic material.

The multilayer plastic pipe E according to the invention shown in FIG.11 comprises an inner pipe Ei, an outer pipe Eo and an intermediatelayer Ef made of foamed plastic.

The foaming of the plastic material can be improved by adding thereto atmost about 1% of chemical foaming agent. It is also possible to addstaple fibres, which can be placed in an angular position deviating fromthe central axis of the pipe, in the plastic material forming the innerand outer pipe and possibly also the intermediate layer. It should benoted that it is not possible to circulate with conventional extruderspolypropylene that is reinforced with glass and that is used widely forexample in car industry to be used in pipe arrangements, since in theseextruders the fibres are oriented along the pipe axis, so that theimpact resistance properties of the pipe deteriorate. In order to avoidthis drawback, it has previously been necessary to separate the fibresfrom the matrix or to grind them into almost round fillers. A possiblealternative or complementary process to the use of staple fibres orfibre-like fillers is the orientation of the plastic material of theinner and/or outer pipe with the method according to the presentinvention.

At least one layer (either the inner pipe Ei, the outer pipe Eo or theintermediate layer Ef) consists of fibre-containing solid or foamedplastic. The plastic can be any extrudable polymer. It has surprisinglybeen found that a very good pipe can be manufactured from a mixture ofpolypropylene and wollastonite that comprises about 20% by weight ofwollastonite. When this material mixture is foamed into an intermediatelayer, the result is a foamed pipe that is stronger than usual. Thefibre-like mineral particles of wollastonite probably reinforce the foamoptimally. When recycled materials are used, the plastic may bepolypropylene that contains glass fibres and it may also contain rubberthat is obtained from the reclaiming of car tyres. The reprocessedplastic may also contain fibering compatibilizer plastic, such as liquidcrystalline polymer LCP. The solid outer surfaces and/or theintermediate layer Ef that is formed of fibre-containing reprocessedplastic preferably comprise a diagonal orientation field. Asreinforcement can also be used recycled cross-linked plastics or fibresif they are first grinded into powder before extruding them with thematrix plastic.

The inner pipe Ei may also be made of medium-density polyethylene (MDPE)of pressure pipe quality, or of some other thermoplastic orthermosetting plastic. The density is typically 940 kg/m³ and the wallthickness is 1 to 5 mm, most suitably 2 to 3 mm.

The outer pipe Eo may correspondingly be made of high-densitypolyethylene (HDPE) intended for example for blow moulding and having adensity of about 955 kg/m³ and a wall thickness that substantiallyequals the wall thickness of the inner pipe Ei.

The density of the intermediate layer Ef is considerably lower than thatof the inner and outer pipe Ei, Eo and the wall thickness is typically 5to 20 mm, preferably about 10 mm. An extremely good product is obtainedwhen the intermediate layer consists of cross-linked polyethylene or amixture thereof. The weight of the foamed layer is typically over 50% ofthe mass of the entire pipe. Even though the foaming degree were high,the foam containing oriented fibres provides excellent strengthproperties. With the method according to the invention, it is alsopossible to prepare foamed pipes where the weight percentage of thefoamed layer of the entire mass of the pipe is substantially smaller,without the economy of the pipe deteriorating.

Especially the inner pipe Ei, but possibly also the outer pipe Eo, isreinforced with fillers or fibrelike reinforcements and/or thepreparation material of the pipe is oriented. The pipe has very greatimpact strength if the inner and/or outer pipe are made of orientedthermoplastic which comprises as reinforcement broken glass fibres thatare not oriented in the axial direction of the pipe.

It is also preferable that the inner pipe Ei is made of eithernon-pigmented or lightly pigmented plastic, whereupon possible damagescan be easily located by means of a video check. The outer pipe Ei haspreferably been UV-stabilized, normally by pigmenting it black, so thatthe pipe stands well for example storing outside, and the soot blackcolour also increases the impact resistance of the outer pipe.

It has been observed that solid state grinded PEX is very efficientreinforcer because of its platelike or fibrelike characteristics. Micais also very efficient platelike reinforcer acting at the same time asaxial reinforcer and as barrier layer. Therefore an advantageouspressure pipe could be made by making the inner pipe Ei of cross-linkedPE and the intermediate layer Ef of polyolefin reinforced with mica.

FIG. 12 shows a detail of the end of the supply conduit. The referencenumerals in FIG. 12 correspond to those in FIGS. 1 to 11. The supplyconduit 6 is formed in such a way that the material to be supplied isdivided in the supply conduit 6 into two different material flows 34aand 34b. The material flows 34a and 34b are supplied to different placesaccording to FIG. 12, thus avoiding the agglomeration of the materials.

The drawings and the description related thereto are only intended toillustrate the inventive idea. The details of the invention may varywithin the scope of the claims. Therefore it is possible to use forexample only one stator outside of which there is a rotor and inside ofwhich there is also another rotor whereby for example referring to FIG.1 reference numeral 1 would be a stator and means referred withreference numerals 2 and 3 would be rotors. Essential to the inventionis however that there is at least two annular conical feed gaps.

What is claimed is:
 1. A method for producing homogeneous material withan extruder comprising at least two annular conical feed gaps situatedbetween a rotatable rotor and a stator so that the material to beextruded is supplied between the rotor and the stator, said methodcomprisingintroducing an extrudable material to said extruder, wherein,between the delivery point of the material to be extruded and the nozzleof the extruder in the direction of travel of the material, the at leastone rotor or stator of said extruder is provided with openings passingtherethrough so that at least some of the material to be extruded ismade to flow through the openings from one annular conical feed gap toanother annular conical feed gap; and extruding said material.
 2. Amethod according to claim 1, wherein several different materials aresimultaneously supplied to the extruder.
 3. A method according to claim1, wherein at least some of the material is supplied to a distance fromthe wide end of the rotor and conducted thereafter to the wide end ofthe rotor and made to flow via the openings to the other side of therotor and conducted further to the narrow end of the rotor and out ofthe nozzle of the extruder.
 4. A method according to claim 3, wherein inconnection with the openings there is at least one discharge outlet fordischarging gas and moisture from the material to be extruded.
 5. Amethod according to claim 4, wherein the gas and moisture are dischargedvia the discharge outlet through the material to be supplied to theextruder.
 6. A method according claim 3 wherein the extruder comprisesat least two rotors, and that the material to be supplied is fed bothbetween the inner rotor and the inner stator and between the outer rotorand the outer stator.
 7. A method according to claim 6, wherein a partof the material to be supplied between the inner rotor and the innerstator is conducted between the inner rotor and the inner stator towardsthe wider end of the inner rotor, and a part of the material isconducted between the inner rotor and the inner stator towards thenarrower end of the inner rotor, and a part of the material to besupplied between the outer rotor and the outer stator is conductedbetween the outer rotor and the outer stator towards the wider end ofthe outer rotor, and a part of the material is conducted between theouter rotor and the outer stator towards the narrower end of the outerrotor.
 8. A method according to claim 7, wherein the part of thematerial that is supplied between the inner rotor and the inner statorand conducted between them towards the narrower end of the inner rotorforms the inner skin of the plastic product, and that the part of thematerial that is supplied between the outer rotor and the outer statorand conducted between them towards the narrower end of the outer rotorforms the outer skin, that the material to be supplied between the innerrotor and the inner stator and conducted between them towards the widerend of the inner rotor and the material to be supplied between the outerrotor and the outer stator and conducted between them towards the widerend of the outer rotor form the intermediate layer of the product, thatthe material to be supplied between the inner rotor and the inner statorand conducted between them towards the wider end of the inner rotor isfed between the inner rotor and the inner stator from the inside of theextruder, whereupon the material is warmed by the waste heat of theextruder, and that the inner skin, the outer skin and the intermediatelayer are formed in the same thermic unit.
 9. A method according toclaim 1, characterized in that the material is supplied between therotor (3, 3a) and the inner stator (1) from the inside of the extruder.10. A method according to claim 1, characterized in that the material tobe supplied is fed into the exturder along a supply conduit in such away that the material is divided in the supply conduit at least into twodifferent material flows that are supplied to different parts of theextruder.
 11. An extruder comprising at least two annular conical feedgaps situated between a rotatable rotor and a stator, and means forsupplying the material to be extruded between the rotor and the stator,wherein, between the delivery point of the material to be extruded andthe nozzle of the extruder in the direction of travel of the material,the at least one rotor or stator comprises openings passingtherethrough, so as to enable at least some of the material to beextruded to flow through the openings from one annular feed gap toanother annular feed gap.
 12. An extruder according to claim 11, whereinat least one stator and at least one rotor (3, 3a-3c) comprise notchesfor mixing the material to be extruded, and that the notches provided inthe rotor are positioned to overlap the notches provided in the stator.13. An extruder according to claim 12, wherein the notches are oval inshape and they are placed diagonally in the rotor and the stator.
 14. Anextruder according to claim 11, wherein between the rotor and the statorthere is a gap.
 15. An extruder according to claim 11, wherein theextruder comprises several material supply conduits, whereupon severaldifferent materials can be simultaneously fed into the extruder.
 16. Anextruder according to claim 11, wherein the extruder is placed in such away that at least a part of the material to be extruded is conducted toa distance from the wider end of the rotor, and that the rotor isarranged to transport at least some of the material to the wide end ofthe rotor where the material is made to flow via the openings to theother side of the rotor, and the rotor is further arranged to transportthe material thereafter to the narrow end of the rotor and further outof the nozzle of the extruder.
 17. An extruder according to claim 16,wherein in connection with the openings there is at least one dischargeoutlet for removing gas and moisture from the material to be extruded.18. An extruder according to claim 17, wherein the discharge outlet isarranged to discharge the gas and moisture to be removed via thematerial to be supplied to the extruder.
 19. An extruder according toclaim 16, wherein there are at least two rotors, and that the firstsupply conduit is arranged to supply the material to be extruded betweenthe outer rotor and the outer stator, and the second supply conduit isarranged to supply the material to be extruded between the inner rotorand the inner stator.
 20. An extruder according to claim 19, theextruder comprises a third supply conduit that is arranged to supply thematerial to be extruded between the outer rotor and the outer stator,and a fourth supply conduit that is arranged to supply the material tobe extruded between the inner rotor and the inner stator, and that theinner rotor is arranged to transport the material supplied along thesecond supply conduit between the inner rotor and the inner statortowards the wider end of the inner rotor, and the material suppliedalong the fourth supply conduit between the inner rotor and the interstator towards the narrower end of the inner rotor, and the outer rotoris arranged to transport the material supplied along the first supplyconduit between the outer rotor and the outer stator towards the widerend of the outer rotor, and the material supplied along the third supplyconduit between the outer rotor and the outer stator towards thenarrower end of the outer rotor.
 21. An extruder according to claim 11,wherein the supply conduit is arranged to supply the material to beextruded between the rotor and the inner stator, and wherein the supplyconduit is situated inside the extruder.
 22. An extruder according toclaim 11, wherein the extruder comprises at least one supply conduitthat is placed in such a way that the material to be supplied can bedivided in the supply conduit into two different material flows that canbe supplied to different parts of the extruder.
 23. An extruderaccording to claim 11, wherein the extruder comprises plate springs thatare placed in such a way that the rotor can move in the axial directionof the extruder with respect to the stators.
 24. A multilayer plasticpipe comprising an inner and outer pipe and an intermediate layer,wherein the intermediate layer is formed of softer material and issituated between the inner and outer pipe, and at least one layerconsists of reinforced solid or foamed reprocessed plastic wherein thesolid outer surfaces and/or the intermediate layer formed offiber-containing reprocessed plastic, comprise a diagonal orientationfield.
 25. A plastic pipe according to claim 24, wherein the reprocessedplastic contains rubber obtained from the reclaiming of car tires.
 26. Aplastic pipe according to claim 24, wherein fibering compatibilizerplastic is mixed in the reprocessed plastic.
 27. A plastic pipeaccording to claim 24, wherein at least one layer is cross-linked.
 28. Aplastic pipe according to claim 27, the reinforcement consists ofrecycled cross-linked plastics or fibres.
 29. A multilayer plasticpressure pipe according to claim 27, wherein the inner pipe is ofcross-linked polyethylene and the intermediate layer is of polyolefinreinforced with mica.
 30. A multilayer plastic pipe comprising an innerand outer pipe and an intermediate layer, wherein the intermediate layeris formed of softer material and is situated between the inner and outerpipe, and at least one layer consists of reinforced solid or foamedreprocessed plastic wherein the outer and/or inner pipe comprise stablefibers or wollastonite, the orientation of which differs from the axialdirection.
 31. A plastic pipe according to claim 26, in which thefibering compatibilizer plastic is LCP plastic.
 32. A plastic pipeaccording to claim 24, in which the fiber-containing reprocessed plasticis polypropylene which is enforced with glass fibers.
 33. A plastic pipeaccording to claim 30 wherein the reprocessed plastic contains rubberobtained from the reclaiming of car tires.
 34. A plastic pipe accordingto claim 30 wherein fibering compatibilizer plastic is mixed in thereprocessed plastic.
 35. A plastic pipe according to claim 30 wherein atleast one layer is cross-linked.
 36. A plastic pipe according to claim30 wherein the reinforcement consists of recycled cross-linked plasticor fibers.
 37. A multilayer plastic pressure pipe according to claim 30wherein the inner pipe is of cross-linked polyethylene and theintermediate layer is of polyolefin reinforced with mica.
 38. A plasticpipe according to claim 34, in which the fibering compatibilizer plasticis LCP plastic.